The organic electroluminescent (EL) devices made by using organic substances have been expected to be applied to production of large area full-color display devices of a solid light emission type at low costs, and have been intensively developed. In general, the organic EL devices are constituted from a light emitting layer and a pair of counter electrodes between which the light emitting layer is sandwiched. In the organic EL devices, when an electric field is applied between the electrodes, electrons are injected from a cathode into the light emitting layer, whereas holes are injected from an anode into the light emitting layer. The electrons and holes injected are recombined in the light emitting layer, so that the light emitting layer is brought into a excited state. When the light emitting layer is returned from the excited state to a ground state, an energy is released in the form of light.
The conventional organic EL devices require a high drive voltage and exhibit a low luminance and a low efficiency of light emission as compared to inorganic light emitting diodes. In addition, the conventional organic EL devices suffer from remarkable deterioration in properties and, therefore, are still practically unusable. Although recently developed organic EL devices are gradually improved to some extent, there is a further demand for developing organic EL devices exhibiting a still higher efficiency of light emission and having a still longer life.
For example, there is disclosed such a technique using a single monoanthracene compound as an organic light-emitting material (JP 11-3782A). However, in this technique, a luminance obtained by using the material is as low as 1650 cd/m2, for example, at a current density of 165 mA/cm2, and an efficiency of light emission thereof is very low, i.e., only 1 cd/A, which is practically unusable. Also, there is disclosed a technique using a single bisanthracene compound as an organic light emitting material (JP 8-12600A). However, in this technique, an efficiency of light emission obtained by using the material is also as low as about 1 to 3 cd/A. Therefore, further improvement of the technique has been demanded for rendering it practically usable. On the other hand, there have been proposed long-life organic EL devices using a distyryl compound as an organic light-emitting material to which styrylamine, etc., is added (WO 94/006157). However, the organic EL devices still fail to exhibit a sufficiently long life and, therefore, further improvement of these devices has been required.
Further, there are disclosed techniques using a mono- or bis-anthracene compound together with a distyryl compound in an organic light emitting medium layer (JP 2001-284050A). However, in these techniques, the wavelength of emission spectra is too long owing to a conjugated structure of the styryl compound, resulting in poor color purity of light emitted.
In addition, WO 04/044088 discloses a blue light-emitting device using a diaminochrysene derivative. The device is excellent in efficiency of light emission but still fails to show a sufficiently long life and, therefore, further improvement of the device has been required.